This invention relates to cold-formable, open-celled rigid polyurethane foams, a process for their production and to the use of such foams to make molded articles such as parts for automobiles.
The expression "cold-formable" as used herein means that the foam sheet is not heated before molding, but is instead placed in a heated mold at room temperature and molded shortly afterwards. The heating effect which spreads from the mold to the foam at the moment of molding is negligible because the foam is surrounded sandwich-fashion on both sides by a glass mat and a surface layer and by the adhesive applied to each of these flat materials. The foam is therefore shielded from the hot mold (130.degree. C.) during the closing process.
Rigid polyurethane foams which are heated before molding are known and are described, for example, in German Offenlegungsschriften 2,607,380 and 3,610,961.
Published European Patent Application 0,118,876 describes flexible to semirigid polyurethane foams which are taught to be suitable both for thermoforming and for cold forming. The fact that these foams may be cold-formed is not surprising because they are extremely soft and flexible. This softness and flexibility is attributable to the use of formulations where only 50 to 100 parts by weight MDI (i.e., diphenylmethane diisocyanate) are used for each 100 parts by weight of the described polyol formulations.
The softness of foams such as those described in European Patent Application 118,776 makes it impossible for them to be readily processed to produce car headliners using only glass mats, surface layers and thinly applied adhesive films. When the hot mold is opened, the composite of foam and the supporting (i.e., glass mat) and decorative (i.e., surface) layers bonded thereto is so soft that it cannot be removed from the mold without damage. Consequently, soft foams such as those disclosed in European Patent Application 118,776 could not be used on a semi-automated production line for the production of a few thousand car roofs per day.
The inventors of the foams disclosed in European Patent Application 118,776 recognized this deficiency of soft foams and attempted to offset this disadvantage by enveloping the foam core in unsaturated polyester resin prepreg mats so that the inadequate supporting function of the foam would be compensated by a relatively hard shell construction. However, it is well known that unsaturated polyester resin prepreg mats can cause physiological/ecological problems due to the evaporation of unreacted styrene.
Another disadvantage of foams such as those described in European Patent Application 118,776 lies in their high resilience. These foams are so elastic that they cannot be durably embossed or compressed because the compressed foam expands again. This is undesirable for decorative and design reasons because, for example, the compressed rim of a car roof must retain its narrow cross section to fit into the narrow gap provided in the pillar trim.